Led driver for powering an led unit from an electronic transformer

ABSTRACT

An LED driver comprising a power converter for powering an LED unit and a control unit for controlling the power converter is provided. The power converter comprising an input terminal for receiving a rectified AC supply voltage, and an output terminal for supplying a current to the LED unit, and the control unit comprising an input for receiving a supply signal representative of the supply voltage and an output for providing a control signal to the power converter. The control unit is further arranged to: determine the control signal for controlling the power converter based on the supply signal, and control the power converter to supply the current to the LED unit based on the control signal, the current being amplitude modulated in synchronism or in phase with the rectified AC supply voltage.

FIELD OF THE INVENTION

The present invention relates to LED based lighting applications, inparticular to lighting applications that are powered from an electronictransformer and/or TRIAC dimmer. Such an arrangement is oftenencountered in a retrofit situation whereby a conventional halogen lightis replaced by an LED unit while a power converter such as an electronictransformer is maintained.

BACKGROUND ART

In general, LED based lighting applications (also referred to as LEDunits, comprising one or more light emitting diodes (LEDs)) are poweredby an LED driver (e.g. comprising a switched mode power supply such as aBuck or Boost converter) which is supplied from a DC voltage source. Insuch an arrangement, dimming of the light (in response to a userinterface action) is typically realised by adjusting the duty cycle ofthe LED or LEDs of the application. As such, conventional LED driversare not suited for being powered by a voltage source which differs froma DC voltage source, such as e.g. provided by an electronic transformeror a standard TRIAC dimmer. When an LED driver is supplied from avarying voltage source, the instantaneous voltage available as input tothe LED driver may be momentarily insufficient to power the LED or LEDsof the lighting application. This could result in flicker of thelighting application which could result into a range of effects in anobserver, from awkwardness via irritation to nausea.

When such a varying voltage source such as an electronic transformer ora TRIAC dimmer is used to power a normal halogen light, the powerreceived will be averaged out and will not result in flicker, althougheven with halogen lights, the low output levels are cumbersome andflicker can be seen in many cases.

In case a lighting application is powered from an electronictransformer, it is further required to, in order for the electronictransformer to provide an output voltage for supplying the LED unit,maintain a current as provided by the transformer above a certain level.As such, when the supply current (i.e. the current supplied to the LEDdriver) is insufficient, the electronic transformer will cease toprovide an output voltage. Subsequently, the electronic transformerwill, after a certain amount of time, attempt to resume its properoperation. Meanwhile however, the light output of the LED unit could beinterrupted, whereas a continuous light output would be desired. Inorder to ensure proper operation of the electronic transformer, it hasbeen proposed in literature to provide a load in parallel to the LEDdriver in order to ensure that a minimum supply current is beingsupplied by the electronic transformer. Maintaining such a current mayresult in an Important dissipation, adversely affecting the efficiencyof the lighting application.

Ensuring that a sufficiently high supply current is provided by anelectronic transformer powering one or more LED drivers, is renderedeven more difficult because of the high power to light conversion of LEDbased lighting applications, compared to conventional halogen lights. Aswill be understood, when a 20 W halogen light bulb is replaced by a 5 WLED unit, the power to be supplied by the electronic transformer can bereduced significantly and may result in a supply current insufficientfor proper operation of the electronic transformer.

In view of the above, it is an object of the present invention tofacilitate the powering of LED based lighting applications by anelectronic transformer (optionally preceded by a TRIAC based dimmer),thereby facilitating conventional applications such as halogen lights toretrofit with LED based lighting application.

SUMMARY OF THE INVENTION

According to an aspect of the Invention, there is provided an LED drivercomprising

-   -   a power converter for powering an LED unit;    -   a control unit for controlling the power converter;    -   the power converter comprising        -   an input terminal for receiving a rectified AC supply            voltage, and        -   an output terminal for supplying a current to the LED unit,    -   the control unit comprising    -   an input for receiving a supply signal representative of the        supply voltage and    -   an output for providing a control signal to the power converter,        whereby    -   the control unit is arranged to:    -   determine the control signal for controlling the power converter        based on the supply signal, and    -   control the power converter to supply the current to the LED        unit based on the control signal, the current being amplitude        modulated in synchronism or in phase with the rectified AC        supply voltage.

The LED driver according to the Invention comprises a power converter,such as a Buck, Boost or hysteretic converter and a control unit (e.g. acontroller or microprocessor) for controlling the power converter. Inaccordance with the invention, the control unit controls the powerconverter by providing a control signal to the power converter, thecontrol signal being based on a supply signal that is received at aninput of the control unit.

The control signal may further, in an embodiment, be based on aset-point (e.g. representing a required intensity or colour setting),e.g. received via RF or any other communication means.

In accordance with the invention, the supply signal represents thesupply voltage that is supplied to the power converter. As such, thesupply signal can e.g. be a signal that is proportional to the supplyvoltage (e.g. obtained via an A/D conversion of the supply voltage). Thesupply signal can be derived or retrieve at various positions, e.g. atthe output terminal of the electronic transformer, after a rectificationof the transformer's output voltage, at the input terminals of the LEDdriver, . . . . In accordance with the invention, the control unit isarranged to control the power converter in such manner that an LED unit,in use powered by the LED driver, is not provided with a substantiallyconstant current, rather, the LED unit is, in use, provided with acurrent with varying amplitude, the amplitude variation (or modulation)being in synchronism or in phase with the supply voltage, in use, arectified AC voltage. By modulating the current as supplied to the LEDdriver as stated, it has been observed that, when the rectified ACsupply voltage originates from an electronic transformer, the electronictransformer can more easily sustain the supply voltage. It has thus beenfound that, in order to ensure that an electronic transforms keepsproviding an output voltage, it is not required to maintain the currentsupplied to the LED unit at a constant comparatively high value in orderto sustain the electronic transformer. Rather it has been foundsufficient to provide a comparatively high current to the LED unit whenthe supply voltage is comparatively high. As such, an amplitudemodulation of the current provided to the LED unit in synchronism withthe rectified AC supply voltage facilitates the power source (e.g. anelectronic transformer and/or a TRIAC dimmer) in providing an outputvoltage used for generating the supply voltage of the LED driver. In anembodiment, the current as provided to the LED unit is arranged to varyin phase with the rectified AC voltage.

Assuming a rectified AC voltage having a main frequency component of 100Hz, (e.g. obtained by transforming and rectifying an AC mains supply of230 V, 50 Hz), it has been found sufficient, for most electronictransformers, to ensure that the current as supplied by the transformercomprises a 100 Hz component (or a multiple thereof) substantially inphase with the main frequency component of the rectified AC voltage. Asthe power supplied to an LED unit can be approximated to be proportionalto the current through the LED unit, the average power supplied to theLED unit can be substantially smaller than the peak power which issupplied when the current supplied is at its peak value.

In an embodiment, the LED driver further comprises a rectifier arrangedto receive an AC supply voltage and provide the rectified AC voltage tothe input terminal. The AC supply voltage can e.g. be provided by anelectronic transformer or a TRIAC dimmer modulating an AC supplyvoltage, or a combination thereof.

In an embodiment, the LED driver further comprises an energy storageelement connectable to the input terminal and a switch for connectingand disconnecting the energy storage element to the input terminal, theswitch being controlled by the control unit, based on the input signal.As an example, such an energy storage element can comprise a capacitoror an assembly of capacitors which can be charged (by the supplyvoltage) and discharged (towards the LED driver) when the switch isoperated at appropriate instances.

The application of such an energy storage element can improve theperformance in different ways. As the energy storage element is chargedfrom the rectified AC voltage, e.g. originating from an electronictransformer, the charging current increases the instantaneous currentdemand of the LED driver which can thus facilitate sustaining anelectronic transformer. The energy storage element can further improvethe LED driver's performance by applying it as a power source when thesupply voltage is comparatively low.

As will acknowledged by the skilled person, in order to provide acurrent to an LED unit, a minimal voltage (known as the forward voltageVf) is required in order to provide a current to the LED unit. Dependingon the type of power converter applied, supplying such a minimal voltageat the output terminal of the power converter may equally require aminimal voltage at the input terminal of the power converter. When thisvoltage is not available, the power converter cannot supply the requiredcurrent to the LED unit. However, in case e.g. a charged capacitor (e.g.charged to a voltage level corresponding to the peak value of therectified AC voltage) would be available, this capacitor could beapplied, temporarily, as a supply source, thereby improving the currentsupply towards the LED unit. In the absence of an energy storage elementthat can be applied as a temporary power source, the current supplied tothe LED unit could reduce to zero during part of the period of therectified AC supply voltage. Depending on the main frequency of thesupply voltage, this could be observed by a user or could even result inthe user experiencing nausea. The application of an energy storageelement as described also enables to adjust the frequency content of thecurrent supplied to the LED unit, thereby mitigating any adverse effectssuch as flicker. In case a comparatively large amplitude modulationwould be required to sustain an electronic transformer to provide thesupply voltage, this could e.g. result in the current provided to theLED unit comprising a comparatively large 100 Hz component. Such an 100Hz component could be undesired for certain observers. By applying anenergy storage element for providing a current to the LED unit when thesupply voltage is comparatively low, the frequency content of thecurrent to the LED unit can be altered. By introducing current peaks(e.g. by discharging a charged capacitor) when the supply voltage iscomparatively low, the main frequency component of the current suppliedto the LED unit can become a 200 Hz current Instead of a 100 Hz current.In general, due to the application of the switchable capacitor (ingeneral, the energy storage element), which can be applied as a voltagesource when the rectified AC voltage is comparatively low, a currentcomponent at twice the main frequency of the rectified AC voltage (e.g.a 200 Hz current in case the rectified AC voltage originates from a 50Hz mains supply) can be introduced in the current as supplied to the LEDunit. By doing so, adverse effects such as an observable flicker ornausea can be reduced significantly.

In addition to (or as an alternative to) the application of an energystorage device, the LED driver according to the invention can beprovided with a power factor correction device. Various embodiments ofsuch a power factor correction device are discussed in more detailbelow. In an embodiment, the power factor correction device can beapplied as an energy storage device, e.g. comprising one or morecapacitances.

In an embodiment, the power factor correction device can be connectedand disconnected from the input terminal via a switch that is controlledby the control unit of the LED driver, e.g. in accordance with thesupply signal. Connecting and disconnecting the power factor correctiondevice can thus be synchronised with the rectified AC supply voltage.

The control unit of the LED driver according to the invention can, in anembodiment, be arranged to determine a minimum value for the amplitudemodulation in order to sustain the supply voltage. This can e.g. be doneby starting with a comparatively large amplitude modulation, graduallyreducing the amplitude modulation applied to the current, monitor if thesupply voltage is sustained, and, if the supply voltage is no longersustained, gradually increase the amplitude modulation until the supplyvoltage is sustained again.

By doing so, the current variation, and any possible adverse effects ofit, can be reduced to a minimum while sustaining the electronictransformer.

In this respect, it is worth noting that the required amplitudemodulation (required to sustain an electronic transformer supplying theLED driver) can depend on the total load to be powered by thetransformer. In case a single transformer is used to provide a supplyvoltage to a plurality of LED drivers, the required amplitude modulationcan be comparatively small or even zero, compared to the case wherebythe transformer only supplies a single LED driver.

With respect to maintaining an electronic transformer to supply anoutput voltage, it is worth noting that the amplitude modulationrequired to sustain the transformer, may depend on the maximum amplitudeof the supply voltage as provided. As will be understood by the skilledperson, this maximum amplitude may vary in time, e.g. due to loadchanges in the electric grid supplying the electronic transformer. Assuch, it may be required to increase the amplitude modulation or theamplitude of the current profile as provided to the LED driver, when thesupply voltage maximum amplitude increases, in order to sustain theelectronic transformer. As in general, there is a limited number ofcurrent levels available that can be selected (e.g. 16 current levelsranging from zero to 120% of the nominal current). If the level of thecurrent supplied to the LED unit would be raised by one level, such achange would become visible to an observer. Instead of applying such asudden current increase, it is proposed in the present invention togradually raise the average current level of the current supplied to theLED unit. This can be realised by raising the current supplied to theLED unit to the next available current level for only a comparativelysmall portion of a period of the rectified AC voltage. This smallportion can e.g. correspond to T1=1/F whereby F represents the frequencyat which a new current set-point can be provided to the LED driver. Incase a new current set-point can e.g. be provided every 52 μsec, (T1=52μsec), the average current over a period of the supply voltage could beincremented in very small steps by increasing the current during eachperiod of the supply voltage only over a period equal to T1, rather thanadjusting (raising or decreasing) the current profile entirely to a nextcurrent level. Phrased differently, the current as provided to the LEDunit can e.g. have a staircase profile, ascending when the supplyvoltage increases and descending when the supply voltage decreases. Thelevels of the staircase would thus correspond to the available currentlevels. Instead of incrementing each level of the staircase profile withone level, the average level is gradually increased by increasing onlyone level (or part of one level, e.g. only during a period T1) of thestaircase profile with one current level. By doing so, the resolution atwhich the average current can be varied is increased significantly,compared to a resolution solely based on the available number of currentlevels.

The LED driver according to the present invention thus enables thepowering of a comparatively low number of LED units by an electronictransformer even if the average power to the LED units is lower than aminimum power requirement of the transformer. This facilitates theapplication of the LED driver according to the invention in retro-fitsituation. It is further worth noting that the LED driver according tothe invention may also be applied when the supply voltage is providedfrom a conventional magnetic transformer, which e.g. merely transforms a230V, 50 Hz mains voltage to a suitable lower voltage by an inductivecoupling. In case the supply voltage originates from a magnetictransformer, there is no need to perform the amplitude modulation andconventional current control can be applied by the LED driver. In suchcase, it may however still be advantageous to apply an energy storageelement to avoid a visible flicker of the LED unit's light output.Therefore, in an embodiment, the LED driver according to the inventionis arranged to detect what type of transformer (either a conventionaltransformer or an electronic transformer) is providing the supplyvoltage. This can e.g. be realised by applying a rapid currentfluctuation (i.e. a comparatively large increase or decrease of thecurrent provided to the LED unit) and monitoring the effect of suchcurrent fluctuation on the supply voltage. It has been devised by theinventors that the application of a rapid current fluctuation duringeither the ascending or descending slope of the supply voltage, canresult in an electronic transformer ceasing to provide an outputvoltage. Because a conventional transformer is not or hardly affected bysuch a current fluctuation, distinction can be made between aconventional transformer and an electronic transformer providing thesupply voltage.

The following figures provide further details of embodiments of thepresent invention whereby corresponding reference numbers indicatecorresponding features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an output voltage as can be obtained froman electronic transformer.

FIG. 2 schematically depicts an electronic transformer scheme.

FIG. 3 schematically depicts a lighting application including an LEDdriver according to the Invention.

FIG. 4 schematically depicts an embodiment of an LED driver according tothe invention.

FIG. 5 schematically depicts a minimum current requirement inrelationship with a supply voltage waveform as obtained from anelectronic transformer.

FIGS. 6a-6b schematically illustrate an operation of an LED driveraccording to the invention.

FIGS. 7a and 7b schematically illustrate an operation of an LED driveraccording to the invention when a TRIAC dimmer is applied for providingthe supply voltage.

FIGS. 8a and 8b schematically indicate how to gradually increase theprofile of the current as supplied to the LED unit.

FIG. 9 schematically depicts a current profile as can be applied todetermine whether or not an electronic transformer is providing thesupply voltage.

FIGS. 10a-10c schematically depict several embodiments of a power factorcorrection device as can be applied in an LED driver according to theinvention.

DESCRIPTION

At present, lighting applications such as halogen lights are oftensupplied from an electronic transformer or a conventional magnetictransformer. In the latter case, a mains input voltage (e.g. 230V, 50Hz) is converted to a comparatively low voltage, e.g. 12V, 50 Hz. Incase an electronic transformer is applied, a mains AC voltage (or aTRIAC dimmer output voltage) is modulated (e.g. at 50 or 60 kHz) to avoltage form as schematically shown in FIG. 1 which is scaled down tothe appropriate voltage level using a transformer. Due to the highfrequency content of the modulated voltage, the volume of thetransformer can be reduced significantly, compared to a transformeroperating at 50 or 60 Hz. FIG. 2 schematically depicts an electronicscheme of an electronic transformer as e.g. can be applied to provide asupply voltage to an LED driver according to the invention or can beapplied in a conventional manner to e.g. supply one or more halogenlights. The electronic transformer e.g. comprises a transistor pair Q1,Q2 arranged to, in use, ensure that an alternating voltage (as e.g.shown in FIG. 1) is available at output terminal ‘out’ of the electronictransformer.

In order to remain providing a supply voltage (as e.g. shown in FIG. 1)at the terminal ‘out’ a minimum current should flow through thesecondary winding (5-6) of transformer TR2. This current results in aproportional current in the primary winding (1-4). Because the primarywinding is series connected with the primary winding (1-4) oftransformer TR1 with two secondary windings (5-6 and 7-8) connected toresp. bases of the transistors Q1 and Q2, said transistors providing theoscillation, the voltage drop over the windings (5-6 and 7-8) willbecome too low when the current as supplied to the load becomes too low.As a result, transistors Q1 and Q2 will no longer switch and theoscillation will cease. As soon as the voltage over capacitor C4(sustained during the oscillation via D1) has become sufficiently lowdue to a discharge over R7, Q3 will cease to conduct thereby forcing Q2to conduct, via R9, D2, D3. When, at that time (i.e. when Q2 start toconduct), a sufficiently large current can flow, the oscillation willresume. Otherwise, it will extinct again. By Q2 starting to conduct, avoltage will appear over R2 which, via D1 is transferred to C4. Q3 willthus conduct thereby delaying a subsequent triggering of the oscillationover some time (typically 400-500 microsecond).

The output voltage can subsequently be rectified to obtain a rectifiedAC supply voltage which can be provided at an input terminal of an LEDdriver according to the invention. The rectifier providing the AC supplyvoltage may also be implemented as part of the LED driver.

Conventionally, the output voltage of the electronic transformer can beapplied to supply a halogen light. In such a situation, in order toproperly operate, the power rating of the electronic transformer shouldmatch the power requirements of the halogen light that is poweredbecause, as is known to the skilled person, an electronic transformerrequires a minimum load in order to keep providing an output voltage. Aswill be illustrated in the following figures, when a supply current ofan electronic transformer drops below a certain value, the outputvoltage cannot be maintained and gradually reduces to zero.Subsequently, a start-up circuit of the electronic transformer (e.g. thecircuit associated with the transistor Q3 as shown in FIG. 2) willattempt to resume proper operation. Such an attempt may e.g. occur,depending on the type of electronic transformer, every 0.5 ms. Once theload requirements for the electronic transformer are sufficient tosustain the output voltage, the electronic transformer will resume itsproper operation.

In case of an LED based lighting application, the output voltage of theelectronic transformer can be applied to supply a power converter of anLED driver. Examples of such power converters are Buck or Boostconverters. In order for such an LED driver to power an LED unit,similar constraints with respect to load requirements have to be met, inorder for the electronic transformer to keep providing a supply voltageto the power converter.

In accordance with the present invention, a strategy has been devisedwhich enables an LED based lighting application having a power ratingbelow the power rating of an electronic transformer, to be powered bysuch a transformer, substantially without any noticeable intensityvariations. In order to achieve this, an embodiment of the LED driveraccording to the invention is controlled in such manner that a currentcan be supplied to a LED unit, essentially in an uninterrupted manner.

In case an LED driver or a power converter of an LED driver is suppliedfrom a varying supply voltage such as a rectified AC-voltage or avoltage originating from an electronic transformer (optionally precededby a TRIAC dimmer), it may be advantageous or even required to providethe LED driver with an energy storage element. Such an energy storageelement can e.g. comprise one or more capacitors connectable orconnected to the input terminals of the power converter of the LEDdriver. In such case, the one or more capacitors can be charged by thesupply voltage when this voltage is comparatively high and discharged,thereby supplying the power converter of the LED driver, when the supplyvoltage is comparatively low.

FIG. 3 schematically depicts such an arrangement. In FIG. 3, referencenumber refers to an AC supply voltage (e.g. a 230 V, 50 Hz mains supplyvoltage) which is applied as an input voltage for an electronictransformer 20. The electronic transformer 20 can be provided with arectifier or can be followed by a rectifier (not shown) resulting in arectified AC voltage at the output terminals 22. Note that the rectifiercan also be provided as part of the LED driver. In order to filter highfrequency components in the rectified AC voltage (e.g. due to themodulation of the AC supply voltage 10 by the electronic transformer20), a filter capacitor 50 can be provided. As such, a filtered,rectified AC voltage can be obtained as a supply voltage 24 for an LEDdriver 30. As schematically shown, the LED driver 30 comprises acapacitor 60, which operates as an energy storage element. The capacitor60 can e.g. be charged by the supply voltage 24 thus resulting in anadditional voltage source which can be used for supplying the LED driverin case the supply voltage 24 is comparatively low. In an embodiment,the connection of the energy storage element 60 to the output voltage ofthe electronic transformer or the LED driver can be controlled. As anexample, a (electronic) switch (not shown) can be provided in serieswith the energy storage element 60 whereby the switch can be opened whenthe capacitor 60 is charged and is subsequently closed in order tosupply the LED driver, when the supply voltage 24 is low. In such anarrangement, a diode (not shown) can be connected between the outputterminal 22 and the energy storage element to ensure that power can onlybe supplied to the LED driver. FIG. 3 further schematically shows an LEDunit 40 being powered by the LED driver 30.

In order to ensure the operation of an electronic transformer providinga supply voltage, at least during part of the period of the supplyvoltage, the LED driver according to the present invention is arrangedto draw a current from the supply voltage, the current being amplitudemodulated in synchronism or in phase with the rectified AC supplyvoltage. This will be explained in more detail below. By applying anenergy storage element such as the capacitor 60, the variation of thecurrent as supplied to the LED unit (which could be noticed by someobservers), can be mitigated. The use of such an energy storage elementas a temporary supply source also enables the frequency content of thecurrent as supplied to the LED unit to be raised. As an example, byappropriate switching of the switched capacitor (as is explained in moredetail below) and thus applying the charged capacitor as a voltagesource when the supply voltage Is comparatively low, the current assupplied to the LED unit may have a main frequency component at twicethe main frequency of the rectified AC voltage (i.e. a 200 Hz currentcomponent in case the rectified AC supply voltage originates from a 50Hz mains voltage and thus has a main frequency component of 100 Hz).

In case a switchable storage element is used, the control unit of theLED driver can be arranged to control both the current as provided tothe LED unit and the switching of the switchable storage element suchthat the total current as provided by the power supply (e.g. the sum ofthe load current of the storage element and the current provide to theLED unit) is synchronised or in phase with the rectified AC supplyvoltage.

As an alternative or in addition to the application of a switchablestorage element, the load of the LED driver according to an embodimentof the invention as perceived by the electronic transformer providingthe supply voltage, can gradually be raised thereby facilitating theproper operation of the electronic transformer. Gradually increasing theload as perceived by the electronic transformer e.g. be done byconnecting one or more comparatively small capacitors as an additionalload to the LED driver. In an embodiment, such an arrangement of e.g. ncapacitors provided with a switch to selectively connect the capacitorscan be integrated in the LED driver. In practice, a single electronictransformer is often applied to power a plurality of LED units (or LEDdrivers). It has been observed by the inventors that care should betaken not to add too much load to the LED driver or drivers, as this maycause damage to the electronic transformer. When the power drawn by theLED units (or the LED drivers powering the LED units) is too small, theoutput voltage of the electronic transformer can drop to zero. In orderto sustain the output voltage of the transformer, the control unit ofthe LED drivers according to an embodiment of the invention can bearranged to increase the power consumption of the LED drivers, by addingan extra load. However, as it may be a-priori unknown how many LEDdrivers (say in a case: N) are powered from a single electronictransformer, the additional load may rise to N times the load which isminimally necessary for a particular type of electronic transformers toremain outputting power. This follows from the observation that such anadditional load can only be determined at design-time of the LED driver.With certain types of load (f.e. capacitive loads), adding acomparatively high load may damage the electronic transformer, thuslimiting N to only 1 or 2 nodes. By making the LED driver adapting tothe situation, that is to the number of LED drivers N, a situation canbe reached whereby only the bare minimum of additional load is addedover the entire system (i.e. over all N LED drivers) to sustain theelectronic transformer.

Assuming the additional load needed to keep the electronic transformeroperating to be a capacitor of X nF. In case more than one LED unit ispowered from the transformer, it may be sufficient to add to eachlighting application a load which Is only a fraction of X nF, namelysubstantially 1/N times X nF. As N is a priori unknown, it is proposedaccording to the invention, to gradually increase the additional loadwhereby an assessment is made whether the added load is sufficient, eachtime a load, e.g. X/Y nF is added. Each time a load is added, theelectronic transformer will, as indicated above, attempt to output poweragain. In case the load of the transformer is insufficient, thetransformer will cease to output power indicating that further additionsof the load are required. As such, it may typically take a few periodsbefore the total added load by the N LED drivers equals or exceeds theminimal extra load. As an example, assuming a minimal load requirementto be 15 nF whereby the load represented by each LED driver can beincreased in steps of 2 nF during each period. In such a situation, itwould take three periods to obtain or exceed the minimal load when 6 LEDdrivers are powered by the transformer. In case 10 LED drivers arepowered, it would only take one period to obtain or exceed the minimalload. As soon as the minimal load required is added, the lightingapplications can stop adding load. Using this approach, one can avoidthat the total load to be powered by the electronic transformerincreases to a level that would cause damage to the electronictransformer. As indicated above, the application of an additional loadcan be combined with the application of an LED current being amplitudemodulated in phase with the rectified AC supply voltage. Due to theadditional load, a smaller amplitude modulation may be applied tosustain the transformer during part of the supply voltage period,compared to the situation where no additional load is applied.

An example of an LED unit being powered by an LED driver isschematically shown in FIG. 4. FIG. 4 schematically depicts an LEDdriver comprising a power converter 150 arranged to power an LED unit(110,120,130) and a control unit 140 arranged to control the powerconverter and, in the embodiment depicted, also the LED unit. The LEDunit (110,120,130) comprises a serial connection of three units 110, 120and 130. The embodiment further comprises a switch assembly comprisingthree switches T1, T2 and T3 that can substantially short circuit therespective units 110, 120 and 130. The switches can e.g. comprise a FETor a MOSFET. FIG. 4 further depicts a power converter 150 for poweringthe LED units and a control unit 140 for controlling the power converter150. The power converter can e.g. be, as shown in FIG. 4, a buckconverter or can be another type of converter that enables theapplication of a current I to the LED unit. The power converter 50 issupplied from a voltage source V, e.g. a rectified AC voltage sourceobtained by rectifying an electronic transformer output voltage. Inaccordance with the invention, the control unit 140 is provided with asignal 160, the signal representing the supply voltage V that isprovided to the converter 150. As further shown in FIG. 4, the controlunit 140 can further be equipped to provide an On/Off signal to theconverter 150 in order to turn the current source on or turn it off. Inaccordance with the invention, the control unit 140 is further arrangedto control the power converter 150 by providing a control signal S tothe power converter. The control signal can e.g. be applied by the powerconverter to control the switching element T of the converter therebycontrolling the current I as supplied by the power converter to the LEDunit. As such, the control signal can e.g. comprise a current set-pointfor the power converter whereby the power converter controls the dutycycle of the switching element in order to obtain the required currentset point. In order to achieve this, a voltage over resistance Rs can beapplied as a feedback to the control unit 40 and to the converter 150(inputted at a terminal FB of the converter), the voltage representingthe current through the LED unit and can thus be applied to control theswitching element T of the converter, e.g. based on a difference betweenthe required current (represented by the control signal S) and theactual current (represented by the voltage over resistance Rs). The LEDdriver as schematically shown in FIG. 4 further comprises a switchablestorage element (or switchable energy storage element) connected to therectified AC supply voltage. The switchable storage element comprises anenergy storage element 155.1 (e.g. a capacitance or Inductance) and aswitch 155.2 controlled by the control unit 40, e.g. based on the signal160 representing the rectified AC supply voltage.

In an embodiment, the energy storage element 155.1 may also function asa power factor correction device. In the embodiment as shown, this wouldthus result in a switchable power factor correction device. As analternative, a static power factor correction device can be applied incombination with the switchable storage element as shown. Embodiments ofa (switchable) power factor correction device are explained in moredetail below.

Conventionally, the current I as supplied by the power converter to theLED unit or the LEDs of the LED unit is kept at a nominal value. Inorder to change an intensity of the light emitted, the duty cycle atwhich the current is provided, is changed, e.g. by operation of a switchin parallel with the LED unit or an LED of the LED unit (such asswitches T1, T2 and T3) as shown in FIG. 4.

With respect to the application of an electronic transformer as a supplysource for an LED driver (or a power converter of the LED driver), ithas been devised, according to the invention, that such application canbe facilitated by applying a varying current I to the LED unit. Morespecifically, it has been devised by the inventors that by applying anamplitude modulation to the current supplied, the amplitude modulationbeing in phase with the supply voltage (i.e. the rectified AC voltage),the proper operation of the electronic transformer can be more easilysustained. Within the meaning of the present invention, an amplitudemodulation in phase with the supply voltage can be described as, but isnot limited to:

-   -   The current as provided to the LED unit comprising a frequency        component in phase with a main frequency component of the supply        voltage. As an example, the current as provided to the LED unit        can comprise a 100 Hz component in case the rectified AC supply        voltage originates from a 50 Hz mains supply voltage, the 100 Hz        current component being in phase with the main (100 Hz)        component of the rectified voltage.    -   The current as provided to the LED unit comprising a frequency        component in constant phase relationship with a main frequency        component of the supply voltage. As an example, the current as        provided to the LED unit can comprise a 200 Hz component in case        the rectified AC supply voltage originates from a 50 Hz mains        supply voltage, a peak of the 200 Hz current substantially        coinciding with a peak of the rectified voltage.    -   The current as provided to the LED unit being above a certain        level when the supply voltage is above a specific value or a        specific percentage of the supply voltage peak value. As an        example, the current as supplied to the LED unit can be        block-shaped switching between a first, comparatively high,        level (e.g. 120% of the nominal current) and a second,        comparatively low, level (e.g. 20-30% of the nominal current),        whereby the first current level Is applied when the rectified AC        supply voltage is above a specific value.

FIG. 5 schematically illustrated a possible way of modulating thecurrent supplied to the LED unit, in order to sustain the electronictransformer. FIG. 5 schematically depicts (graph a) a rectified ACvoltage 200 as e.g. obtained as an output of an electronic transformer.Graph b schematically depicts a current profile 210 that enables, whensuch a current is drawn from the supply, the electronic transformer tomaintain providing an output voltage. As such, this profile can also bedescribed as a minimal required current for sustaining the transformer.In the example as shown in FIG. 5, the required minimal current 210 canbe considered to vary proportional, or in phase, with the voltage 200 assupplied to the LED driver. In case the rectified AC voltage originatesfrom a 50 Hz mains supply, the required current profile would thuscomprise a 100 Hz component, 100 Hz also being the main frequency in therectified AC voltage as provided to the LED driver. As can be seen whencomparing both graphs a and b of FIG. 5, the minimum current required tosustain the electronic transformer is comparatively high when the supplyvoltage is high and can be comparatively low when the supply voltage islow. In order to realise such a current profile, the supply current Isto the LED unit can e.g. be controlled at a level above the nominalcurrent (e.g. 120%) when the supply voltage is high and controlled to alevel below the nominal current (e.g. 80%) when the supply voltage islow. Such a current profile 220 is schematically depicted in graph c ofFIG. 5 together with the minimum current 210.

In accordance with the present invention, the current as provided to theLED unit need not necessarily be in phase with the rectified AC supplyvoltage, as it may be sufficient to synchronise the current as provideto the LED unit with the rectified AC supply voltage. By synchronisingthe current to the LED unit, the supply voltage (e.g. provided by anelectronic transformer or a TRIAC dimmer) used as input for providingthe rectified AC supply voltage can at least be sustained for aconsiderable part of a period of the supply voltage. As such, during acomparatively small part of the period of the supply voltage, the supplyvoltage may reduce to zero, the current to the LED unit thus beingreduced to zero as well. For some application, having the LED currentreduce to zero during a comparatively small part of the period of thesupply voltage may be acceptable. In case this is not acceptable, anembodiment of the LED driver according to the invention is provided withan energy storage element (such as a capacitance) which can be used, asexplained in more detail below, as a power supply for generating an LEDcurrent when the supply voltage is absent.

With respect to the required current profile as depicted in graph b ofFIG. 5, it is worth mentioning that such a profile, in particular thelow current portion of the profile, may be difficult, if not impossible,to realise when the supply voltage is low. In particular, in order tosupply a current to an LED unit by an LED driver, a minimum inputvoltage needs to be available at the LED driver (the minimum voltagebeing related to the forward voltage requirements of the LED unit). Thisminimum voltage level is schematically indicated in graph a of FIG. 5using reference number 205. As such, when the supply voltage 200 is lessthan the minimum voltage 205, the LED driver cannot supply a current tothe LED unit. When no current is (temporarily) provided to the LED unit,the minimum current level 210 is not realised, consequently, theelectronic transformer would cease to provide the supply voltage.

In order to overcome this, an energy storage element such as a capacitorcan be applied in an embodiment of the LED driver according to thepresent invention, to supply the LED driver when the supply voltage islow. The application of an energy storage element is illustrated belowusing one or more capacitors. The same principles as explained below canhowever also be implemented when one or more inductances are applied asenergy storage elements.

In order to charge the capacitor, different approaches can be applied,as illustrated in FIG. 6a . Graph a of FIG. 6a schematically shown (insolid thick line) the voltage as available at the input terminal of theLED driver when a capacitor is appropriately connected and disconnectedto the supply voltage (i.e. a rectified AC voltage).

In an embodiment, the capacitor is charged in a continuous mode (seegraph b) thereby connecting the capacitor to the input terminals (i.e.to the rectified AC supply voltage) until the capacitor is substantiallycharged. In such mode, the capacitor can e.g. remain connected to thesupply voltage until the voltage reaches its maximum. At this point, thecapacitor will be charged entirely and can be disconnected. By doing so,the capacitor remains charged to this maximum voltage. In graph b thecapacitor is connected to the supply voltage from instance 510 andremains connected, e.g. until the supply voltage reaches its maximumvalue, instance 501. Graph b shows the supply current Is as drawn fromthe supply voltage. Is comprises a component Iload, the current assupplied to the LED unit and a component due to the charging of thecapacitor. As from instance 502, the capacitor is reconnected and theavailable voltage (at a level 505) can be used to supply the loadcurrent to the LED unit. Consequently, the supply current Is can reduceto zero. At instance 503, the supply voltage becomes sufficiently high,compared to the voltage available at the capacitor, for the electronictransformer to restart and provide the supply voltage. As from thatinstance, the capacitor can be charged again.In an embodiment, illustrated in graph c of FIG. 6a , the capacitor (orcapacitors) is charged in a pulsed manner. By sequentially connectingthe capacitor to the supply voltage for only a (very) short time, thecapacitor is gradually charged by a number of current pulses which will,in general, have a comparatively high amplitude (due to the differencebetween the supply voltage and the capacitor voltage when usingsufficiently small connection times) and can be used to sustain theelectronic transformer even when the load current supplied to the LEDunit is smaller than the minimum current requirement of the transformer.Graph c schematically depicts the current is as drawn from the supplyvoltage comprising a component Iload, the current as supplied to the LEDunit and a component due to the charging of the capacitor, i.e. acomponent consisting of a number of current pulses. The duty cycle atwhich the capacitor is connected and disconnected is schematicallyindicated above graph c.

The operation of an embodiment of the LED driver according to theinvention is further illustrated in FIG. 6b . The LED driver is assumedto be supplied from a rectified AC voltage originating from anelectronic transformer at its Input terminals. In the embodiment asshown, a switchable capacitor connectable between the input terminals ofthe LED driver (see further in e.g. FIG. 9) is further assumed, thecapacitor thus being arranged to be charged from the rectified ACvoltage from the electronic transformer. The capacitor can be used, whenat least partly charged, to supply the LED driver.

In FIG. 6b , graph a (solid thick line) schematically depicts thevoltage as can be supplied to the LED driver by properly charging anddischarging the capacitor. During operating, a distinction can be madebetween the following operating modes, referred to as charge, run andboost. When operating in the charge mode, the capacitor is connected tothe supply voltage such that it is charged by drawing a current from theelectronic transformer. Such a connection can e.g. be established byclosing a switch connected in series with the capacitor. Such a switch,e.g. a FET or a MOSFET, can be controlled by the control unit of the LEDdriver, e.g. based on a signal representing the supply voltage availableat the input terminals. By having the capacitor charged by theelectronic transformer, maintaining the supply current of thetransformer above a minimum value is facilitated. As such, during thecharging of the capacitor, the current as supplied to the LED unit canbe lower than the minimum current while maintaining the electronictransformer operative.

When the capacitor is disconnected from the supply voltage, the LEDdriver is operated in the run-mode. During this mode, the LED unit ispowered by the electronic transformer. In order to keep the electronictransformer providing the supply voltage, the current as supplied to theLED unit should thus be larger or equal to the minimum current. In orderto realise this, the LED unit can be controlled to operate above itsnominal current during this mode, i.e. during the run-mode. In general,the run-mode starts when the capacitor is disconnected from the supplyvoltage (this disconnection preferable occurs when the supply voltage isat its maximum value) and ends when the capacitor is re-connected.

When the capacitor is reconnected to the LED driver, the chargedcapacitor can be applied as a voltage source for powering the LEDdriver. This mode of operation is referred to as the boost-mode.

In an embodiment, the boost-mode is started when the supply voltage asprovided by the electronic transformer to the LED driver is too low topower the LED unit. As will be understood by the skilled person, inorder to supply a current to an LED unit, a minimum voltage equal to therequired forward voltage of the LED unit needs to be available. Based onthe topology of the LED unit and the specifications of the LED or LEDsapplied, a control unit of the LED driver can determine the requiredminimal voltage that should be available at the LED driver inputterminals in order to supply a current to the LED unit. When theavailable voltage would become insufficient to power the LED driver, thecontrol unit of the LED driver can thus control the switch associatedwith the capacitor thereby connecting the charged capacitor to the inputterminals of the LED driver. When operating in the boost-mode, e.g.starting from instance 502, the LED driver is supplied from the chargedcapacitor (in general, the energy storage element). Supplying the LEDdriver from the charged capacitor enables powering the LED driver andthus providing the LED unit with a current. Note that, in the absence ofthe charged capacitor, no power could be delivered to the LED unit whenthe supply voltage is below the required forward voltage (indicated bydotted line 505) of the LED unit. As such, no current would be drawn bythe LED driver and the electronic transformer would cease providing thesupply voltage. When the charged capacitor is applied to power the LEDdriver during the boost-mode, the electronic transformer will also ceaseto provide the supply voltage. However, such an interruption of thesupply by the electronic transformer can remain unnoticed due to thepower supply by the charged capacitor. As such, by applying an energystorage element as a supply source during part of the period of therectified AC voltage (e.g. when the voltage is below a certain level),the electronic transformer need not be sustained during that part andcan cease to provide an output voltage. When the electronic transformerhas stopped providing an output voltage, the transformer will attempt,e.g. every 0.5 ms, depending on the type of transformer, to restore theoutput voltage again. Such attempt will fail however as long as theelectronic transformer cannot supply a current to the LED driver. Aslong as the output voltage is smaller than the available voltage overthe capacitor, an attempt to restart the transformer will thus fail.When the output voltage exceeds the voltage available at the capacitor,the electronic transformer can resume supplying a current to the load(i.e. the LED driver powering the LED unit) and to the capacitor,thereby charging the capacitor. In graph d of FIG. 6b , the chargingcurrent is indicated as 532, whereas 531 indicates the current to theLED unit, 533 indicates the sum of 532 and 531.

In practice, as indicated in graph e, a larger margin between therequired current 540 and the actual current 542 supplied to the LED unitcan be applied. When a sufficiently high supply voltage is available,the current supplied to the LED unit 542 can e.g. be above the nominalvalue (e.g. at 120%) and decrease below the nominal value at otherInstances. Similar to curve 531 of graph d, the current 542 comprises acomponent substantially in phase with the main frequency component ofthe rectified AC supply voltage (i.e. a 100 Hz component in case thesupply voltage originates from a 50 Hz mains supply). In addition,current 542 comprises current peaks 544 occurring at times when thesupply voltage is comparatively low, i.e. when the LED driver issupplied from the energy storage element. By doing so, a currentcomponent at twice the frequency of the main component of the rectifiedAC voltage (e.g. a 200 Hz component in case the supply voltageoriginates from a 50 Hz mains supply) is introduced. By doing so,adverse effects of the intensity variation of the LED unit can bemitigated.Graph f finally describes the current Is as provided by the electronictransformer 550, together with the minimum current requirement 510. Ascan be seen, when the current 550 drops below the minimum current 510(because the charged capacitor has taken over supplying the loadcurrent), the current rapidly drops to zero, due to the electronictransformer stopping. At times when the electronic transformer is notproviding an output voltage, the LED driver can rely on the energystorage element (e.g. a charged capacitor or capacitors) to provide therequired input power to supply a current to the LED unit.

In order to synchronise the operation of a switch connecting ordisconnecting a capacitor to the supply voltage, a reference instancecan be determined relative to the period of the rectified AC voltage.The timing of the operation of the switchable energy storage element canthen be controlled by the control unit, relative to the referenceinstance. Given the reference Instance, the peak value of the supplyvoltage and frequency, the control unit can determine at each instancethe available supply voltage and thus determine whether or not tooperate the switchable energy storage element. As a reference instance,the control unit can e.g. determine (during a number of periods of thesupply voltage), when the voltage is reduced by e.g. 3 or 5% compared tothe peak value. This is schematically illustrated in FIG. 7a for asupply voltage that is phase angle modulated, e.g. by a TRIAC dimmer. InFIG. 7a , the dotted line 700 schematically indicates a rectified ACvoltage whereas thick solid line 710 indicates a phase angle modulated(by phase angle α) AC voltage as can be obtained for a leading edgeTRIAC dimmer. Also indicated in FIG. 7a is instance tm whereby thevoltage 710 is at its maximum and instance tr (the reference instance),e.g. corresponding to a voltage that is 5% less than the maximumvoltage. The reference instance that enables a synchronisation of theswitching of an energy storage element, may also be applied to determinethe phase angle modulation α when such a modulation is applied, e.g. bya TRIAC dimmer applying a leading or trailing edge phase modulation tothe supply voltage or the mains voltage supplying the electronictransformer. The phase angle modulation α, can e.g. be determined fromthe reference instance tr and the instance at which the electronictransformer is successfully started again, corresponding to instance taas indicated in FIG. 7a . In case of a leading edge dimmer, the instanceta could indicates the availability of a sufficiently high supplyvoltage, thus enabling the electronic transformer to power the LEDdriver. In case of a trailing edge dimmer, the instance ta wouldcorrespond to the instance at which the electronic transformer stopsproviding an output voltage. Based upon the phase angle modulation thusdetermined, the control unit can control the average current as suppliedto the LED driver thereby mimicking the conventional use of the dimmer.

When a phase angle modulated supply voltage (as shown in FIG. 7a ) iscombined with the application of a switchable capacitor, a voltageprofile as shown in FIG. 7b can be made available at the terminal of theLED driver. Such a profile can be realised, similar to the profile shownin graph a of FIG. 6b , by appropriate control of the switchablecapacitor, thus operating in either the boost (B), run (R) or charge(C)-mode as described above. The obtained voltage profile can be appliedby the LED driver to supply a current to the LED unit, whereby thecurrent can be amplitude modulated as e.g. described above. As such, thecurrent as provide to the LED unit can e.g. comprise or consist of acurrent component in phase with the main frequency component of therectified supply voltage, or can comprise or consist of a component attwice the main frequency component of the rectified supply voltage.

In an embodiment, the LED driver according to the invention is arrangedto gradually increase the average current to the LED unit when anincrease in the amplitude of the supply voltage is noticed. Such anIncrease can be due to load changes in the electric grid supplying anelectronic transformer supplying the LED driver. Such change in thesupply voltage amplitude is in general, a phenomenon that occurs on acomparatively large time scale (-minutes). A change in the amplitude ofthe available voltage may however affect the required current suppliedto the LED unit in order to sustain the transformer. As such, thecurrent supplied to the LED unit may need to be changed (e.g. increased)when the supply voltage changes (increases). In accordance with theinvention, such an increase is done gradually, in order for the changein brightness (due to the change in current) to remain unnoticed to theobserver. Assuming that a limited number of current levels is availablethat can be selected (e.g. 16 current levels ranging from zero to 120%of the nominal current). If the level of the current supplied to the LEDunit would be raised by one level, such a change would become visible toan observer. In accordance with the invention, a gradual increase of thecurrent is realised by raising the current supplied to the LED unit tothe next available current level for only a comparatively small portionof a period of the rectified AC voltage. This small portion can e.g.correspond to T1=1/F whereby F represents the frequency at which a newcurrent set-point can be provided to the LED driver, or a largerportion. In case a new current set-point can e.g. be provided every 52μsec, (T1=52 μsec), the average current over a period of the supplyvoltage could be incremented in very small steps by increasing thecurrent during each period of the supply voltage only over a periodequal to T1, rather than adjusting (raising or decreasing) the currentprofile entirely to a next current level. This is illustrated in FIGS.8a and 8b . In FIGS. 8a and 8b , graph 700 indicates the current profileas applied at a certain period of the supply voltage (indicated by thedotted line). In FIGS. 8a and 8b , period T1 as described above, isindicated. As can be seen when comparing FIGS. 8a and 8b , in order togradually increase the current to the LED unit, the current is raised tothe level indicated as 710 somewhat sooner (over a period T1 sooner),thereby realising an incremental Increase in the average current (seenover one period) and thus resulting in an incremental increase inbrightness which will remain unnoticed by an observer. This process canbe repeated gradually, thereby effectively rendering the current profilesomewhat wider and taller.

With respect to the current profile as schematically depicted in FIGS.8a and 8b , it can further be noted that such a profile can becharacterised by the current slope being equal or larger than zero whenthe rectified AC voltage is ascending, the current slope being equal orsmaller than zero when the rectified AC voltage is descending. It hasbeen observed that applying such a profile further facilitatessustaining an electronic transformer supplying an output voltage. Assuch, it has also been determined by the inventors that the applicationof a current profile which does not comply with this characteristic(i.e. the current slope being equal or larger than zero when therectified AC voltage is ascending, the current slope being equal orsmaller than zero when the rectified AC voltage is descending), cantrigger the electronic transformer to stop providing an output voltage.Therefore, in an embodiment of the present invention, the control unitof the LED driver is arranged to control the power converter of the LEDto supply a current to the LED unit, the current comprising a rapidcurrent fluctuation as e.g. shown in FIG. 9. The current profile 800 asshown in FIG. 9 comprises a current fluctuation on the descending partof the profile. As can be seen, the current profile 800 shows anincrease in current at instance 810 rather than a decrease during thedescending part of the rectified AC voltage. Applying such a profilecan, as has been observed by the inventors, trigger an electronictransformer to stop supplying an output voltage. As such, applying sucha profile enables the control unit to assess whether or not anelectronic transformer is providing the supply voltage.

Referring to FIGS. 3 and 4 above, the LED driver according to thepresent invention can be provided with a power factor correction device.Such a device can e.g. be arranged at the input terminal of the LEDdriver and can be used to improve the power factor of the load (i.e. thepower converter+LED unit of the LED driver).

In FIG. 10a , a first embodiment of a power factor correction device isschematically shown. The power factor correction device as shown in FIG.10a comprises a capacitance network comprising capacitances 901 and 903and further comprises diodes 902, 904 and 905 and an optional resistance905. Reference numbers 900 denote the terminals between which therectified AC supply voltage (e.g. voltage V of FIG. 4) is supplied. Thepower factor correction device can be connected/disconnected from therectified AC supply voltage by controlling the gate 911 of electronicswitch 910, e.g. a MOSFET having an internal diode 912. Duringoperation, the capacitances 901 and 903 can be charged by the rectifiedAC supply voltage via diode 905. The capacitances thus being seriesconnected during charging. Once charged, the capacitances can bedischarged (capacitance 901 can be discharged via diode 902, capacitance903 can be discharged via diode 904), by doing so, the capacitances aredischarged in parallel.

It is worth noting that the power factor correction device as shown mayalso be applied without the electronic switch 910, as schematicallyshown in FIG. 10b . In FIG. 10b , the power factor correction device asshown in FIG. 10a is shown in a static configuration, i.e. without theswitch 910 connecting the device to the terminals 900. As such, thepower factor correction device remains connected between the terminalsat all times. In addition, FIG. 10b schematically shows a furthercapacitance 920 which can be used as an energy storage element which canbe connected/disconnected to and from the terminals 900 by switch 910.The switch 910 as shown in FIGS. 10a and 10b can e.g. be controlled bythe control unit of the LED driver according to the invention in orderto connect and disconnect the power factor correction device orcapacitance 920 at the appropriate instances, which can e.g. be derivedfrom a supply signal, representing the rectified AC supply voltage, thatis provided to the control unit.

In FIG. 10c , another embodiment of a power factor correction device isschematically depicted, the device being connected between terminals1000 representing the rectified AC supply voltage. In the embodiment,the power factor correction device comprises a capacitance 1030 that isseries connected to a parallel arrangement of resistance 1010 and diode1020. The device may in a controlled manner be connected anddisconnected by switch 1040, e.g. an electronic switch such as a FET orMOSFET. During operation, capacitance 1030 can be charged via resistance1010, while discharging can take place via diode 1020. As an alternativeto the parallel arrangement of the resistance 1010 and diode 1020, thepower factor correction device may comprise a current source orinductance arranged in series with the capacitance 1030.

With respect to the use of a power factor correction device as describedabove, it can be mentioned that the application of such a device canresult in the LED driver operating at an improved power factor. Theapplication of such a device may however also be considered as it canallow the profile of the current to the LED unit to be altered. In theabsence of a power factor correction device, particular requirements canbe posed upon the current profile in order to obtain a power factor thatis sufficiently high; as an example, it may be required to have asufficiently large current component in phase with the supply voltage.By using a power factor correction device, the requirements for thecurrent profile can become less strict which can result in an improvedIllumination quality; e.g. less flicker.

It should further be mentioned that the embodiments of the LED driversas described are mere illustrations of the various aspects of theinvention, the invention only being limited by the scope of the claimsas set forth.

1. An LED driver comprising a power converter for powering an LED unit;a control unit for controlling the power converter; the power convertercomprising an input terminal for receiving a rectified AC supply voltageoriginating from an electronic transformer, and an output terminal forsupplying an output current to the LED unit, the control unit comprisingan input for receiving a supply signal representative of the supplyvoltage and an output for providing a control signal to the powerconverter, whereby the control unit is arranged to: determine thecontrol signal for controlling the power converter based on the supplysignal, and control at least one of the power converter and the LED unitto draw an input current from the input terminal, the input currentcomprising a pulsed current, wherein the input current prevents theelectronic transformer from ceasing to supply the rectified AC supplyvoltage.
 2. The LED driver according to claim 1, wherein the controlunit is arranged to control the power converter to draw the pulsedcurrent in synchronism with the rectified AC supply voltage.
 3. The LEDdriver according to claim 1, further comprising an energy storageelement connectable to the input terminal.
 4. The LED driver accordingto claim 3, whereby the energy storage element comprises a capacitance.5. The LED driver according to claim 4, further comprising a firstswitch for connecting and disconnecting the energy storage element tothe input terminal, the first switch being controlled by the controlunit, to draw the pulsed current from the input terminal.
 6. The LEDdriver according to claim 5, wherein the energy storage element isarranged to supply the power converter when the rectified AC supplyvoltage is comparatively low.
 7. The LED driver according to claim 4,wherein the control unit is arranged to operate the power converter in afirst mode, thereby charging the capacitance from the rectified ACsupply voltage; in a second mode, thereby discharging the capacitanceand providing a capacitance discharge current to the power converterfor, at least partly, supplying the LED unit.
 8. The LED driveraccording to claim 7, wherein the capacitance is charged in a pulsedmode, to draw the pulsed current.
 9. The LED driver according to claim8, wherein the control unit is arranged to control the first switch tocharge the capacitor in synchronism with the rectified AC supply voltageto draw the pulsed current.
 10. The LED driver according to claim 9,wherein the control unit is configured to control the first switch tocharge the capacitor when the rectified AC voltage is comparatively highand is configured to operate the power converter in the second mode whenthe rectified AC voltage is comparatively low.
 11. The LED driveraccording to claim 8, wherein the capacitance comprises a plurality ofcapacitances that are sequentially charged.
 12. The LED driver accordingto claim 1, further comprising a power factor correction deviceconnectable to the input terminal.
 13. The LED driver according to claim12, wherein the power factor correction device is connectable to therectified AC voltage via a second switch, the second switch being,controlled by the control unit, based on the input signal.
 14. The LEDdriver according to claim 1, wherein the control signal represents acurrent set point to be followed by the power converter, the current setpoint being amplitude modulated based on the supply signal.
 15. The LEDdriver according to claim 1, wherein a peak value of the input currentis, in use, varied according to the amplitude of the rectified AC supplyvoltage.
 16. The LED driver according to claim 15, wherein the peakvalue of the input current is varied proportional to the amplitude. 17.The LED driver according to claim 15, wherein the variation is appliedgradually by only adjusting a current level during a comparatively smallpart of a period of the rectified AC supply voltage per period of therectified AC supply voltage.
 18. The LED driver according to claim 1,wherein the input current comprises a staircase-like profile.
 19. TheLED driver according to claim 1, wherein the control unit is arranged tocontrol the power converter to apply a current fluctuation during anascending part or a descending part of the rectified AC supply voltagein order to detect whether or not the supply voltage originates from anelectronic transformer.